This invention relates to praseodymium oxide loaded on a high surface area cerium oxide or cerium-zirconium oxide useful as catalysts or catalysts supports.
Materials which can alternately take up and release oxygen, i.e., provide oxygen storage, are useful in a number of applications including selective oxidation and oxidative dehydrogenation of organic compounds, separation of oxygen from air, and cryogenic refrigeration. Oxides of lanthanide elements which have variable oxidation states, cerium (Ce), praseodymium (Pr), and terbium (Tb), are particularly useful for such catalyst since changes in oxidation state, and thus oxygen content, are relatively easily effected. The increase of atomic number in the sequence Cexe2x80x94Prxe2x80x94Tb is accompanied by a corresponding decrease of the stability of MO2 (M=Ce, Pr, Tb) oxides, which in turn reduces the temperature at which oxygen can become available. Thus, in this regard, praseodymium oxide can be considered to be a superior oxygen storage material as compared to cerium oxide.
Cerium oxide, supported on high-surface area alumina at a loading of up to 20 wt %, has long been employed in commercial automotive three-way catalysts primarily as an oxygen storage material. Despite the fact that praseodymium oxide is able to provide oxygen at a lower temperature, praseodymium oxide has not been commercially used in place of the cerium oxide on the alumina. This is due to the fact that praseodymium oxide reacts with the alumina at relatively low temperatures (600xc2x0 C.), forming an aluminate which is ineffective for oxygen storage.
Recently, new automotive catalyst formulations have been developed in which cerium is deployed as part of a mixed oxide, primarily with zirconium, for use as a high surface area catalyst or as a support for a catalyst like platinum. These mixed oxides, e.g., of cerium oxide and zirconium oxide, can be formed by techniques such as mechanical mixing, coprecipitation, or sol-gel processing. The latter is disclosed in EPO patent application 0611192, published Aug. 17, 1994, which teaches forming cerium zirconium mixed oxides from a mixture of a zirconium sol and a cerium sol. A small amount of dopant such as silicon or praseodymium is suggested for inclusion into the sol mixture to act as a stabilizer for the mixed oxides product. As such, the amount of praseodymium oxide is too small to contribute much to the actual oxygen storage capacity. In U.S. patent application Ser. No. 08/650,244 filed May 20, 1996, now abandoned, commonly assigned with the present invention and having a common inventor, a sol-gel process is disclosed for making a high surface area praseodymium-zirconium-oxide useful as a catalyst and a washcoat.
Although the precise role of zirconium in such mixed oxides is poorly understood, it is generally thought that it serves to both promote the oxygen storage function. of cerium oxide and provide thermal stability to the mixed oxide so that it retains it high surface area. Compared with cerium oxide having a thermally stable high surface area, the cerium-zirconium oxide provides superior oxygen storage capacity.
Both of these cited application processes involves using complex sol-gel techniques. It would be desirable to use a less complex method of making a mixed oxides having high surface area and excellent oxygen storage properties. The present invention fulfills such requirements.
The invention is a mixed oxide oxygen storage material consisting essentially of praseodymium oxide loaded onto a high surface area (i) cerium oxide or (ii) cerium-zirconium oxide, the molar ratio of praseodymium to cerium in the mixed oxide being 1:4 to 4:1. Loading of either material with praseodymium oxide may be performed by, for example, standard incipient wetness techniques. The resultant mixed oxide may be used as a catalyst or a catalyst support. In the latter instance, a catalyst material such as a noble metal may be loaded on these mixed oxides. The present invention mixed oxide is thermally stable and maintains a high-surface-area at elevated temperatures as may be experienced in automotive exhaust gas systems.
According to still another embodiment, it is a method for treating automotive internal combustion engine exhaust gases by exposing the exhaust gases to the invention mixed oxides disclosed above, wherein the mixed oxides may be loaded with a catalyst material like palladium.
The above-mentioned interaction problems between praseodymium oxide and alumina can be avoided if the praseodymium oxide is loaded instead on cerium oxide or cerium-zirconium-oxide rather than on alumina. The oxygen storage capacity of the praseodymium oxide loaded onto cerium-zirconium oxide material increases significantly, generally by a factor of 2-5, as compared to the cerium-zirconium-oxide itself. Unexpectedly, we found that there is a synergistic enhancement of oxygen storage capacity in our new praseodymium-cerium-zirconium oxide over that of the sum of praseodymium oxide and cerium-zirconium oxide. And, praseodymium oxide loaded onto a high surface area cerium-zirconium oxide was found to be thermally stable and able to maintain its high surface area at elevated temperatures. Furthermore, praseodymium oxide when supported on high-surface-area cerium oxide likewise provides a new oxygen storage material with thermal stability and oxygen storage capacity which is comparable to that for praseodymium oxide supported on cerium-zirconium oxide. Hence, the present invention thermally stable, high-surface area mixed oxides allow praseodymium oxide to be deployed as a catalyst or catalyst support in which its superiority over purely cerium-based oxides as oxygen storage materials is maintained. The increase in oxygen storage capacity of the mixed oxides believed attributable to the praseodymium oxide is found to be especially enhanced by high-temperature treatment of the mixed oxide as might be experienced in an automotive exhaust gas system.